The beam from the telescope passes through an interface unit, the FASU (Filter and Spectral Lamp Unit), which is mounted between the instrument mount cube and the main instrument. The FASU contains two wheels, FASU-A and FASU-B (in the telescope converging beam) for narrow band filters and the calibration mirror. The interface unit also houses the calibration lamps for wavelength calibration and flat-fielding. Both wheels in the FASU have 5 slots each. After the beam has passed the FASU, it comes to focus on the aperture wheel which is located at the focal plane of the telescope. The aperture wheel has slots in which various slits can be mounted. The beam then passes through the collimator and forms a parallel beam. The filter wheel and the girsm wheel, each having eight slots, are placed in the parallel beam, in between the collimator and the camera. The parallel beam is imaged by the camera on the CCD. All HFOSC wheels have 8 positions.
| Position | slit |
| 1 | 134 l |
| 2 | 1340 l |
| 3 | 167 l |
| 4 | 335 l |
| 5 | 100 m |
| 6 | 67 l |
| 7 | 67 s |
| 8 | free |
| Position | Filter/Grism |
| 1 | free |
| 2 | Gr. 14 |
| 3 | Bes I |
| 4 | Bes R |
| 5 | Bes V |
| 6 | Bes B |
| 7 | Bes U |
| 8 | Gr. 11 |
| Position | Grism |
| 1 | focus pyramid |
| 2 | Gr. 8 |
| 3 | Gr. 9 |
| 4 | Gr. 7 |
| 5 | Gr. 5 |
| 6 | Gr. 17 |
| 7 | Gr. 15 |
| 8 | free |
| Position | Filter |
| 1 | free |
| 2 | 486.1 |
| 3 | 656.3 |
| 4 | Cal-Mir |
| 5 | 500.7 |
| Position | Filter |
| 1 | free |
| 2 | 672.4 |
| 3 | Halpha (broad) |
| 4 | 372.7 |
| 5 | free |
| Width | Height | |
| micron | arcsec | arcmin |
| 67 | 0.77 | 10 (arcsec) |
| 67 | 0.77 | 11 |
| 100 | 1.15 | 2.1 |
| 100 | 1.15 | 11 |
| 134 | 1.54 | 10 (arcsec) |
| 134 | 1.54 | 11 |
| 167 | 1.92 | 11 |
| 335 | 3.85 | 11 |
| 1340 | 15.41 | 11 |
| Grism # | Wavelength range | Resolution |
| nm | ||
| Gr. 5 | 520 - 1030 | 870 |
| Gr. 7 | 380 - 684 | 1200 |
| Gr. 8 | 580 - 835 | 2000 |
| Gr. 9 (echelle) | 330 - 1050 | 4000 |
| Gr. 10 (cross disp) | 330 - 650 | 230 |
| Gr. 11 (cross disp) | 370 - 740 | 390 |
| Gr. 12 | 520 - 1040 | 205 |
| Gr. 14 | 327 - 616 | 1320 |
| Gr. 15 | 330 - 600 | 745 |
| Gr. 17 | 760 - 1050 | 2640 |
| Filter | |
| Bessell U | |
| Bessell B | |
| Bessell V | |
| Bessell R | |
| Bessell I | |
| Bessell I_c | |
| Bessell z | |
| Halpha Broad(50) | |
| 372.7 (10) | |
| 486.1 (10) | |
| 500.7 (10) | |
| 656.3 (10) | |
| 672.4 (10) |